How Do You Explain the Tides in 10 Seconds?

How do you explain ANYTHING in just 10 seconds? Well, it’s pretty tough. Of course, you can’t tell the whole story in just 10 seconds (unless maybe you talk super fast). Well, this is just what Henry Reich of MinutePhysics is trying to do. I wonder if he knows that a minute is actually 60 seconds and not 10. Oh well. It’s sort of like a the physics video version of a haiku.

This is true. The farther you are from a massive object, the less the gravitational force. So, the side of the Earth that isn’t facing the moon has a lower gravitational force from the moon than the side facing the moon. It’s easy to claim that this causes the tides. It’s simple and easily digestible.

However, the Earth has TWO tides. How do you explain the tide on the far side of the Earth?

In the above diagram (which is not even close to the correct scale), you can see both of the water bulges from the tides. There are some other important things in this picture. First, the red dot shows the center of mass of the Earth-moon system. If these two objects (Earth and moon) were the only things in the universe, they would both orbit around this center of mass. Second, the moon AND the Earth are both moving in circular orbits. It just so happens that the Earth’s orbital radius is smaller than the radius of the Earth.

The fact that the Earth is orbiting is important in an explanation of the tides. When an object moves in a circle, it is accelerating. And how do we handle being on an accelerating surface? The best way is to use a fake force? A fake force is a force that we like to add to a situation to account for an accelerating reference frame. Here is a quick example. Suppose you are in an elevator that is accelerating up.

There are only two real forces on you in this case. There is the gravitational force pulling down and the floor pushing up. The gravitational force doesn’t change since your mass doesn’t change. The floor has to push up with a larger force than gravity in order for you to accelerate up. However, in the frame of the elevator, it seems like you are at rest. So, in your mind (and in calculations) you can add this fake force pushing down. With the fake force, the net force is zero and you stay at rest (in the elevator).

The same thing happens on the far side of the Earth. Since the Earth is moving in a circle (due to the orbit of the moon), this part of the Earth is accelerating towards the moon. The fake force for this acceleration would be in the opposite direction as the acceleration, so it would push AWAY from the moon. This is why there is a second tide.

Yes, it’s actually more complicated than that. The point is that it can’t JUST be the differential gravitational force that causes the two tides. Consider the following experiment. Suppose that I take the Earth and the moon and tie a rope to each one like this:

Yes, you would need some serious ropes. But the point is that if the two objects are stationary then all the water on the Earth would be pulled towards the moon. It would just have water on one side. So, it’s not JUST stronger gravity on one side.

But what would the tide be like on different planets or with a different sized moon? I’ll save that for a later post since it will be much more detailed.